The glass industry, in general, is aware that glass forming batch ingredients can be combined with water to form agglomerates which agglomerates may be dried and heated and then charged to a glass melting furnace. It is also recognized in the glass industry that the flue gases emanating from a fossil fuel fired melting furnace can have a significant roll in such processes. That is, instead of wasting energy in such flue gases, including flue gases which have undergone heat exchange in a recuperator or regenerator, as had previously been done by discharging such gases to the ambient, the flue gases may be used as a source of energy. It is also known to minimize pollutant discharge to the atmosphere, and simultaneously beneficially employ such energy, by transferring at least a portion of otherwise wasted flue gas energy to the agglomerated glass batch by direct contact therewith prior to discharge into the melting furnace.
Some of the most economical of such process teachings of the prior art work well for many glass batch formulations but these teachings are entirely unsuitable for an economical, industrial exploitation of such processes as applied to certain other agglomerated glass batch formulations. Such unsuitability is especially acute in instances of agglomerating certain glass batch formulations with water into the form of pellets. Such water containing agglomerates, to which the teachings of the prior art are ill-suited will henceforth be referred to as "hydrologically unstable" agglomerates. The term hydrologically unstable will be subsequently clarified and the term agglomerate includes within its scope any composite, intergral, discrete, self-supporting mass which includes substantially all essential glass forming batch ingredients. Unless the contrary is indicated, the term agglomerates comprehends within its scope extrusions, disks, briquettes, pellets or other discrete geometric shapes. Normally a maximum dimension of either height, length, or width or diameter of such agglomerates will be on the order of one or two inches and specifically with regard to pellets the maximum diameter will be preferably less than one inch and more typically in a size range of about 3/8 of an inch to about 5/8 of an inch.
It should be borne in mind that the glass industry is a highly capital intensive industry generally operating on low profit margins and high production rates. Thus, for any process to merit industrial exploitation in the glass industry such process must be compatible with the economic nature of the industry. Some of the factors involved include the necessity of low capital expenditures for new equipment installations, and a process which has low operating cost. Included in the latter consideration are not only manpower requirements but also, for example, floor space requirements, as such space is at a premium in virtually all glass industry plants, equipment susceptible to minimal breakdown and damage, and equipment utilizing a minimum amount of operating power. In the context of drying and heating water-containing, glass batch agglomerates with flue gases from a fossil fuel fired melting furnace, the most desirable process equipment of the prior art is a shaft type heater, or chamber, i.e., a vertical bed of substantial height, and preferably a bed in which the agglomerates flow downwardly through the chamber and in which the flue gases flow counter-current to the agglomerates, to substantially continuously, in a single processing operation, dry and preheat them.
Some glass batch agglomerates are, however, unstable and substantially continuous drying and heating in a single operation in a vertical bed is not obtainable. When such water-containing glass batch agglomerates, and especially pellets, are processed in direct contact with, for example, flue gases from a glass melting furnace, which flue gases have passed through a dry portion of the bed to preheat them, the wet agglomerate containing portion of the bed, when at a height in excess of a characteristic value, will aggregate into a strong, rather massive, monolithic type structure, or structures, which plug the shaft heater. This unacceptably necessitates shutdown and results during drying and at temperatures which are well below those which would cause the agglomerates to thermally sinter or fuse together. The height at which aggregate formation occurs is greater for a vertical bed dried and preheated with hot, or warm, dry air, or dry combustion products than where the drying and preheating is done with wet air or wet combustion products, like the flue gases from a fossil fuel fired glass melting furnace. If ambient air is heated, or warmed, at constant humidity, or if combustion is practiced with a large excess of air, such gaseous heating medium may typically, for example, have a wet bulb temperature on the order of about 80.degree. F. to 85.degree. F. (26.degree.-30.degree. C.) and perhaps less. In contrast however, the flue gases from a fossil fuel fired glass melting furnace are the products of stoichiometric combustion and are more humid, or wet; typically they have a wet bulb temperature on the order of about 130.degree. to about 140.degree. F. (54.degree.-60.degree. C.) or higher. This indicates that a significant factor in aggregate formation is the psychrometry of the gaseous heating and drying medium, for example the wet bulb temperature. This is not to imply however, that the use of warm, dry air is satisfactory. First of all, relative to the use of a fossil-fuel fired melter, if such warm dry air had to be separately provided it would increase cost and would not be compatible with the purpose of recovering at least some of the energy normally wasted in the furnace flue gases, nor with attempting to remove pollutants from the furnace flue gas. Furthermore, even the use of such heated dry air does not eliminate aggregate formation. For example, it has been found when directly heating a bed of free water containing pellets with a gaseous heating medium comprising combustion products and having a web bulb temperature of about 80.degree.-85.degree. F. by passing such medium through the bed, that a bed of a height of up to about 8 or 9 inches was characterized by the pellets generally remaining as discrete, free flowing individual pellets. Above that height, however, the pellets were aggregated and no longer free flowing. Such occurrence obviously is unsuitable inasmuch as a typical shaft type heater, or vertical bed of agglomerates, must be in excess of several feet, e.g., ten or more, to maintain compatibility with the pull rate on a melting furnace and provide a sufficient retention time so as to be able to preheat the pellets to the maximum temperature possible, but short of causing the agglomerates to sinter together, and also to allow sufficient agglomerate-flue gas contact time to separate pollutants from the flue gases.
Thus, it will be seen from the above that there is a problem in the glass manufacturing industry with the above type indicated processes in that certain types of free water-containing, glass batch agglomerates simply cannot be suitably processed in a vertical bed. In accordance with the present invention, this problem has now been solved and an improved process is provided. Some of the advantageous features of the present improved process include the ability to substantially continuously dry and preheat such glass batch agglomerates, using a vertical bed, while substantially simultaneously recovering pollutants from flue gases of a glass melting furnace in the bed for recycle into the glass melter. With, for example, Na.sub.2 O and B.sub.2 O.sub.3 containing glasses, otherwise potentially wasted boron values are recovered in the vertical bed as a sodium borate, e.g. NaBO.sub.2, and recycled to the melter. This has the advantage of enhancing the quality of the atmosphere and additionally saves on raw material costs. In conjunction with that, the process recovers otherwise wasted energy, reduces the amount of energy which is normally consumed in a glass melter for melting glass batches and will significantly increase furnace throughput per square foot of melter area. In fact, it is not uncommon to double the throughput per square foot of melter area. Furthermore, the improved process recognizes the economic nature of the glass manufacturing industry and provides for a maximization of economic benefit with a minimization of economic detriment in order to obtain that benefit. Thus, for example, the improved process is not highly capital intensive and is characterized by low operating costs. The significance of the above in light of today's economic conditions, will be appreciated by all.
In passing it should be mentioned that realization of the above type advantages, is not limited to the use of a fossil-fuel fired melter. They may, likewise, be obtained in manufacturing glass by using a melter in which the energy is electrically supplied. In the latter instance instead of using melter flue gases, agglomerate drying and preheating is effected by using separately provided products of combustion for the heating medium and, thereby, the amount of more expensive electrical energy needed for the melter is significantly decreased. Corresponding increased throughputs will also be realized. The heating medium is obtained by the combustion, preferably with a substantial stoichiometric excess, e.g. at least about 50%, typically about 50% to about 400%, of air with any suitable fuel such as coal, oil, natural gas, propane, or the like, depending on cost and availability. In some instances it may even be economically expedient that the heating medium be heated air, for example electrically heated air.
The foregoing problem is solved, and the advantageous features attained, by providing for an improvement in glass manufacturing processes of the type comprising combining glass-forming batch ingredients and water into free water-containing glass batch agglomerates, preferably pellets, continuously directly contacting glass batch agglomerates in a vertical bed with flue gases from a glass melting furnace so as to preheat the agglomerates, discharging preheated agglomerates from a lower portion of the bed, charging preheated agglomerates to a melting furnace and melting the charged agglomerates therein. The improvement resides in adapting such processes to the use of free water-containing glass batch agglomerates which are hydrologically unstable and further comprises accumulating a predetermined amount of such agglomerates in a preconditioning chamber, preferably a plurality of separate chambers operated in a sequential, parallel flow fashion, so as to form a preconditioning bed(s) of a predetermined height, directing flue gases from said vertical bed into said preconditioning chamber and passing said gases through said preconditioning bed so as to heat the bed for a sufficient period of time to form a hydrologically stabilized bed of agglomerates and discharging the agglomerates of the hydrologically stabilized bed and supplying them to the vertical bed. Generally, the height of the preconditioning bed or beds will be significantly less than the height of the vertical bed, e.g. typically less than 5% of the height of the vertical bed.
In accordance with another aspect of the present invention, the foregoing advantageous features are obtained by providing an energy efficient, pollution abating, substantially continuous glass manufacturing process comprising, forming separate beds of free water-containing glass batch agglomerates, discharging the agglomerates of the beds, after at least some heating, into a shaft-type preheating chamber having a vertical bed of agglomerates therein, substantially continuously releasing dry, further heated agglomerates from the vertical bed, charging the dry, further heated agglomerates to a combustion-fired melting furnace, melting the agglomerates therein and, substantially simultaneously conveying flue gases from the furnace to the chamber, through the vertical bed therein so as to further heat the agglomerates of said bed, and then from the vertical bed through said separate beds so as to heat the agglomerates of the separate beds. The separate beds will be heated prior to discharging the agglomerates of the respective beds for a sufficient period of time to assure that they will not form process disabling aggregates in the preheating chamber but the period of time will be insufficient to cause the respective separate beds themselves to convert, or cement, into process disabling aggregates. Preferably, the separate beds are sequentially formed and sequentially heated for the prescribed period of time. That is, while an individual bed is treated in a batch-like sequence, the cycles of the respective plural beds will be such that overall, the heating of the separate beds, and discharging, will be generally along the lines of a continuous processing step. Desirably, the separate beds will be disposed downstream of the vertical bed in a general parallel flow arrangement and will be located adjacently upwardly of the preheating chamber with the flue gases from the chamber being conveyed to the beds also in a parallel flow type manner.
In accordance with another aspect of the invention, there is provided a glass manufacturing process comprising forming separate beds of free-water containing, glass batch agglomerates, and, after at least some heating, discharging the agglomerates of said beds, and supplying them into a shaft type preheating chamber having a vertical bed of agglomerates therein, substantially continuously releasing dry, further heated agglomerates from said vertical bed, charging said dry, further heated agglomerates to a glass melter and melting said agglomerates therein, while conveying gaseous combustion products to said chamber, through said vertical bed so as to further heat said agglomerates, and then from said vertical bed through said separate beds so as to heat the agglomerates therein. The gaseous combustion products may be flue gases from a melter, or the melter may be electrically powered and the combustion products provided by burning air and a fuel.
Yet another aspect of the invention provides an improved glass manufacturing process of the type comprising, combining glass forming batch ingredients and water into free water containing glass batch agglomerates, continuously directly contacting glass batch agglomerates in a vertical bed with gaseous combustion products so as to preheat the agglomerates, discharging said preheated agglomerates from a lower portion of said bed, charging said preheated agglomerates to a melting furnace and melting said charged agglomerates therein. The improvement resides in preventing hydrologically unstable agglomerates from forming large (process disabling) aggregates by continuously supplying said hydrologically unstable agglomerates to a preconditioning chamber so as to form a preconditioning bed of progressively increasing height, discontinuing the supply to said chamber and directing said gaseous combustion products from said vertical bed into said preconditioning chamber and passing said gaseous combustion products through said preconditioning bed so as to heat said preconditioning bed of hydrologically unstable agglomerates for a sufficient period of time to form a hydrologically stabilized bed of agglomerates, discharging the agglomerates of said hydrologically stabilized bed into said vertical bed.
Further in accordance with the present invention there is provided an improved glass manufacturing process comprising discharging a supply of free water containing, hydrologically unstable, glass batch agglomerates gravitationally downwardly, discontinuing said discharging at a predetermined interval and intercepting said gravitationally downwardly discharged supply so as to form said supply into a shallow static bed of substantially uniform height, directly heating said static bed so as to remove at least some free water from the agglomerates and convert said bed to a hydrologically stabilized bed, discharging the agglomerates of said stabilized bed and supplying them to a vertical bed, said vertical bed being maintained at a predetermined minimum height, gravitationally flowing the discharged agglomerates generally downwardly through said vertical bed, heating said vertical bed, including said generally downwardly flowing discharged agglomerates to an elevated temperature by direct contact with a gaseous heating medium passing through said vertical bed, supplying said heated flowing agglomerates after flowing through said vertical bed to a glass melting furnace and melting said agglomerates therein.
The present invention, in addition to the previously indicated advantages, provides outstanding pollution-abatement features when the heating medium are the flue gases of a fossil-fuel fired melter. That is, materials in the heating medium, especially flue gases, which normally decrease environmental quality are reclaimed and recycled into the melting operation. Particulates, for example, are separated by a filter-type action. If desired, a cyclone, downstream of the agglomerate beds, may be employed to effect further reclamation of materials. Some materials are reclaimed by an in-situ reaction and some gaseous polluting species, because of the progressive temperature drop of such flue gases during operation, are reclaimed by a condensation type mechanism. Of course, however, the temperature of the gases during operation will not be allowed to drop to the point where water vapor therein will undergo condensation.